The invention relates to a method for objectively evaluating welded bonds produced by tip or lift-off arc ignition welding processes, between a metallic piece, e.g. a welding stud, and a metallic workpiece.
European Patent Specification 0,100,787 discloses a stud welding process with tip arc ignition in which the characteristics of current and voltage over time and the stud movement are recorded, and the pressure of the welding sound is measured. Characteristics of the welding process, such as the moments of tip contact, of tip explosion and of stud shorting, the current maximum, the stud immersion depth and the like, are derived from the detected values. These characteristics are compared to empirically obtained values, and the degree of deviation is used as a basis for judging the quality of the weld.
German Patent Specification 19,530,416 discloses a similar method in which, in addition to the welding current and voltage, the magnetic field surrounding the arc is detected by means of a magnetic sensor. Information concerning the current distribution within the arcing zone and arc blow effects are obtained from the magnetic field measurement.
The known processes require a plurality of different values characteristic of the welding process to be detected and evaluated for drawing conclusions concerning the welding quality. Moreover, the evaluation is restricted to the coincidence between the results of the measurements and values obtained at one or a plurality of preceding test welds, and a plurality of measurement points are employed for determining the degree of coincidence. Due to the measuring equipment and time required, the known methods are expensive and cumbersome.
Since the various parameters measured are interdependent in a complex way, the results obtained are difficult to interpret and only to a limited extent useful. This becomes apparent parent by the fact that recent attempts to improve the reliability of the results take more and more measurements.
The simultaneous evaluation of a plurality of measurements requires well-trained personnel and considerable expenditure concerning the adjustment and monitoring of parameters. This is also one of the reasons why the known methods are unsuited for immediately influencing a current welding process.